Electrochemical Reduction of Metal Oxides

ABSTRACT

A process for selectively forming a morphology of reduced material is disclosed. The reduced material, such as titanium, is formed by a process of electrochemically reducing a metal oxide feed material, such as titania, in a solid state in an electrolytic cell containing molten chlorine-containing electrolyte in the cell. The process for selectively forming the morphology of reduced material includes sintering the reduced material and forming a morphology that is susceptible to subsequent washing of retained electrolyte in the reduced material.

This application is a continuation-in-part of and claims priority to PCT application PCT/AU2005/001135 filed on Aug. 1, 2005 published in English on Feb. 2, 2006 as WO 2006/010229 and to Australian application no. 2004904304 filed Jul. 30, 2004, the entire contents of each are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to electrochemical reduction of metal oxides.

BACKGROUND OF THE INVENTION

The present invention relates particularly, although by no means exclusively, to electrochemical reduction of metal oxide feed material in the form of powders and/or pellets in an electrolytic cell to produce reduced material, namely metal having a low oxygen concentration, typically no more than 0.2% by weight.

The present invention is concerned with minimising the concentration of chlorine in reduced material produced by electrochemical reduction of metal oxide feed material in an electrolytic cell that operates with a chlorine-containing molten electrolyte, such as a CaCL₂-based electrolyte.

SUMMARY OF THE INVENTION

The present invention provides a process for selectively forming a morphology of reduced material formed by a process of electrochemically reducing a metal oxide feed material in a solid state in an electrolytic cell containing molten chlorine-containing electrolyte in the cell that includes sintering the reduced material and forming a morphology that is susceptible to subsequent washing of retained electrolyte in the reduced material.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example, with reference to the accompanying drawings.

FIG. 1 is a micrograph of a cross-section of a reduced pellet produced with the cell operating with an electrolyte temperature of 1000° C.

FIG. 2 is a micrograph of a cross-section of a reduced pellet produced with the cell operating with an electrolyte temperature of 950° C.

FIG. 3 is a micrograph of a cross-section of a reduced pellet produced with the cell operating with an electrolyte temperature of 900° C.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention was made during the course of an on-going research project on electrochemical reduction of metal oxide feed material being carried out by the applicant. The research project has focussed on the reduction of titania (TiO₂).

During the course of the research project the applicant has carried out a series of experiments investigating the reduction of metal oxide feed material in the form of titania in electrolytic cells comprising a pool of molten CaCL₂-based electrolyte, an anode formed from graphite, and a range of cathodes. The CaCL₂-based electrolyte used in the experiments was a commercially available source of CaCL₂, which decomposed on heating and produced a very small amount of CaO.

The applicant operated the electrolytic cells at potentials above the decomposition potential of CaO and below the decomposition potential of CaCL₂.

The applicant found that the cells electrochemically reduced titania to titanium with low concentrations of oxygen, ie concentrations less than 0.2 wt.%, at these potentials. The applicant operated the cells under a wide range of different operating set-ups and conditions.

One issue that has been addressed by the applicant in the research project is the issue of undesirably high concentrations of chlorine in reduced material that is produced by electrochemical reduction of metal oxide feed material in a solid state in electrolytic cells that operate with a chlorine-containing molten electrolyte, such as a CaCL₂-based electrolyte.

By way of example, high concentrations of chlorine in reduced material are undesirable in situations where the reduced material is titanium metal because the chlorine has an adverse effect on the weldability of the titanium.

The applicant has found in a series of experiments that the conditions under which metal oxide feed material is electrochemically reduced in electrolytic cells has a significant impact on the resultant concentration of chlorine in reduced material produced in the process. The conditions include temperature of molten electrolyte and reduction time.

More particularly, the applicant has found that the reduction conditions in electrolytic cells can have a significant impact on the morphology of the reduced material and that by appropriate selection of the conditions it is possible to produce reduced material that has a morphology that is more susceptible to subsequent washing than would otherwise be the case, with a result that the resultant washed reduced material has a lower concentration of chlorine.

The applicant has found that the appropriate selection of reduction conditions in electrolytic cells include conditions that result in sintering of reduced material such that there is phase separation within the morphology of the reduced material, with one phase comprising regions of reduced material and the other phase comprising regions of retained electrolyte, and with the electrolyte-containing regions being interconnected into a continuous network so that the electrolyte can be washed more readily from the reduced material.

As a consequence of the above finding the applicant has realised that, in general terms, controlled sintering of reduced material in or external to an electrolytic cell may be an effective option for minimising the chlorine concentration in reduced material.

According to the present invention there is provided a process for selectively forming a morphology of reduced material formed by a process of electrochemically reducing a metal oxide feed material in a solid state in an electrolytic cell containing molten chlorine-containing electrolyte in the cell, which process includes sintering the reduced material and forming a morphology that is susceptible to subsequent washing of retained electrolyte in the reduced material.

The step of sintering the reduced material may be carried out in the electrolytic cell. Alternatively, the step of sintering the reduced material may be carried out in a separate vessel that is supplied with reduced material from the electrolytic cell.

Preferably the process further includes removing reduced material from the cell or the separate vessel and thereafter washing the reduced material.

The process may be carried out on a batch basis, a semi-continuous basis, and a continuous basis. The process may be carried out as a single stage or a multi-stage process.

Preferably the metal oxide feed material is in a powder and/or a pellet form. Preferably the metal oxide feed material is a titanium oxide. More preferably, the titanium oxide is titania.

Preferably, the electrolyte is a CaCL₂-based electrolyte containing CaO. In the case of a CaCL₂-based electrolyte containing CaO preferably the electrochemical reduction process includes applying an electrical potential across the anode and the cathode that is above the decomposition potential of CaO and below the decomposition of CaCL₂.

According to the present invention there is also provided a process for electrochemically reducing a metal oxide feed material in a solid state in an electrolytic cell containing molten chlorine-containing electrolyte in the cell, which process includes sintering the electrochemically reduced material and forming a morphology that is susceptible to subsequent washing of retained electrolyte in the reduced material.

As is indicated above, the applicant has found that one morphology that is susceptible to subsequent washing of the reduced material is a structure that includes a continuous network of interconnected regions of chlorine-containing electrolyte.

More particularly, the applicant has found that, as viewed in an optical microscope and a scanning electron microscope, the structure is essentially a two phase structure with one phase comprising grains of reduced material having minimal amounts of electrolyte within the grains and the other phase comprising a continuous network of interconnected regions containing electrolyte.

The experimental work carried out by the applicant to date has focussed on evaluating the impact of the temperature of molten electrolyte and the reduction, ie residence time, of reduced material in an electrolytic cell on the subsequent washability of the resultant reduced material. Specifically, the experimental work indicated that the concentration of chlorine in washed electrochemically-reduced material in the form of titanium metal decreased as the temperature of the molten electrolyte in the electrolytic cell increased and as the residence time of the reduced material in the cell increased. Thus, the experimental work indicated that better washability was achieved with reduced material produced at high temperatures and longer reduction times.

The experimental work was carried out on metal oxide feed material in the form of titania that was supplied to the laboratory-scale electrolytic cell as a product with a relatively fine, uniform porous morphology throughout the material (typically 50% pores). The experimental work indicated that molten electrolyte penetrated the above-described porous titania and was retained in the titania and was not readily washable from the titania. The experimental work also indicated that reduction in the electrolytic cell caused a progressive rearrangement of the morphology of the reduced material to the two-phase morphology described above in response to the sintering conditions in the cell and that electrolyte was more readily washable from this reduced material.

The experimental work carried out by the applicant to date included the experiments described hereinafter and with reference to FIGS. 1 to 3 which are micrographs of cross-sections of pellets of reduced titania produced in an electrolytic cell operating with electrolyte temperatures of 1000° C., 950° C., and 900° C., respectively.

EXAMPLE 1

Pellets of titania were electrochemically reduced in a laboratory-scale electrolytic cell under different molten electrolyte temperatures and reduction times. The titania pellets were manufactured from smaller particles of titania. Typically, the titania particles for pellet manufacture ranged from nanometer size up to 15 microns. The pellets had an open connected pore structure with a porosity in the range of 35-60% by volume. Preferably, at least 25 vol.% of the pores had a size of 0.005-10 microns measured by Mercury Intrusion porosimetry.

Pellets were shaped, for example by being slip cast in moulds. A range of different shaped pellets were used. One example of a shape was a disc with a cylindrical side wall and flat top and bottom walls, and with the diameter of the cylinder being considerably greater that the thickness of the disc between the top and bottom walls. One example of a disc is a disc that is 20 mm in diameter and 2 mm thick. In all cases, a maximum dimension of 3.5 mm applied to the minimum dimensions of pellets (and powders) tested.

The cell contained a commercially available source of CaCL₂ that decomposed on heating and produced a very small amount of CaO. The cell was operated at an applied potential of 3 V. This is a potential above the decomposition potential of CaO under the cell operating conditions.

The pellets of reduced titania were removed from the cell after the prescribed reduction times and were washed in pellet form or in a broken up form in deionised water near boiling point for 2-4 hours. Thereafter, the chlorine concentrations of the pellets were measured and the washed pellets were evaluated under an optical microscope. The results of the chlorine concentrations of the reduced pellets are summarised in Table 1 below.

TABLE 1 Temperature (° C.) 900 940 950 950 1000 1000 1100 1150 Time (hrs) 8.5 8.0 6.0 7.5 5.5 6.5 3.5 2.5 Chlorine (ppm) 7200 5500 3950 4000 2100 1900 1600 1100

The above results indicate in general terms that the concentration of chlorine in the washed reduced titania pellets decreased as the molten electrolyte temperature increased.

The micrographs of FIGS. 1 to 3 are cross-sections of the reduced pellets produced with the cell operating with electrolyte temperatures of 1000° C., 950° C., and 900° C., respectively. Each of the micrographs show a morphology that comprises an essentially two phase structure with one phase comprising grains of reduced titania (the lighter coloured parts of the micrograph) and the other phase comprising regions containing electrolyte (the darker coloured parts of the micrograph).

The micrographs indicate the significantly different morphologies of the reduced pellets produced at these electrolyte temperatures. In particular, the micrographs indicate that the two phase structure became increasingly pronounced as the electrolyte temperature increased. More particularly, the micrographs indicate that as the electrolyte temperature increased, the grains of reduced titania had smaller amounts of electrolyte within the grains and the electrolyte-containing phase increasingly formed as a continuous network of relatively large interconnected pores.

EXAMPLE 2

In this example, the effect of reduction time was evaluated at a selected temperature.

Pellets of titania that were prepared as described above were electrochemically reduced in the same laboratory set-up described above. The molten electrolyte temperature was set at 1000° C.

In a first experimental run, a first set of pellets was removed from the cell after 6 hours. In a second experimental run, a second set of pellets was removed after 8 hours. In a third experimental run, a third set of pellets was removed after 12 hours.

The reduced pellets were washed in pellet form or in a broken up form in deionised water near boiling point for 2-4 hours. Thereafter, the chlorine concentrations of the pellets were measured and the washed pellets were evaluated under an optical microscope.

The results of the chlorine concentrations of the pellets are summarised in Table 2 below.

TABLE 2 Temperature (° C.) Time (hrs) Chlorine (ppm) 1000 6 2000 1000 8 2300 1000 12 1400

The above results indicate in general terms that the concentration of chlorine in the washed titanium pellets decreased as the reduction time increased.

Many modifications may be made to the present invention described above without departing from the spirit and scope of the invention. 

1. A process for selectively forming a morphology of reduced material formed by a process of electrochemically reducing a metal oxide feed material in a solid state in an electrolytic cell containing molten chlorine-containing electrolyte in the cell, which process includes sintering the reduced material and forming a morphology that is susceptible to subsequent washing of retained electrolyte in the reduced material.
 2. The process defined in claim 1 that includes sintering the reduced material and forming a morphology that includes a continuous network of interconnected regions of chlorine-containing electrolyte.
 3. The process defined in claim 1 that includes sintering the reduced material and forming a morphology that includes a two phase structure with one phase comprising grains of reduced material having minimal amounts of electrolyte within the grains and the other phase comprising a continuous network of interconnected regions containing electrolyte.
 4. The process defined in claim 1 that includes sintering the reduced material in the electrolytic cell.
 5. The process defined in claim 1 that includes sintering the reduced material in a separate vessel.
 6. The process defined in claim 1 that further includes removing reduced material from the cell or the separate vessel and thereafter washing the reduced material.
 7. The process defined in claim 1 wherein the metal oxide feed material is in one of a powder and a pellet form.
 8. The process defined in claim 1 wherein the metal oxide feed material is a titanium oxide.
 9. The process defined in claim 1 wherein the electrolyte is a CaCL₂-based electrolyte containing CaO.
 10. A process for electrochemically reducing a metal oxide feed material in a solid state in an electrolytic cell containing molten chlorine-containing electrolyte in the cell, which process includes sintering the electrochemically reduced material and forming a morphology that is susceptible to subsequent washing of retained electrolyte in the reduced material.
 11. The process defined in claim 10 wherein, in the case of a CaCL₂-based electrolyte containing CaO, the process includes applying an electrical potential across the anode and the cathode that is above the decomposition potential of CaO and below the decomposition of CaCL₂.
 12. The process defined in claim 10 includes sintering the reduced material and forming a morphology that includes a continuous network of interconnected regions of chlorine-containing electrolyte.
 13. The process defined in claim 10 that includes sintering the reduced material and forming a morphology that includes a two phase structure with one phase comprising grains of reduced material having minimal amounts of electrolyte within the grains and the other phase comprising a continuous network of interconnected regions containing electrolyte.
 14. The process defined in claim 10 that includes sintering the reduced material in the electrolytic cell.
 15. The process defined in claim 10 that includes sintering the reduced material in a separate vessel.
 16. The process defined in claim 10 that further includes removing reduced material from the cell or the separate vessel and thereafter washing the reduced material. 